www.austriamicrosystems.com/AS3977

Revision 3.6

34 - 47

AS3977

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

9.8 Output Frequency Setting

(EQ 3)

Where:

P

is the Predivider division ratio

M

is the Prescaler modulus

N

is the N-counter division ratio

AC

is the A-counter offset, AC determines together with FRAC the actual A-counter setting

FRAC is the Fractional part

ratio P and the setting of the pre scaler modulus M. The setting of these values for the different frequency bands is given in Table 24.

The reference frequency should be in the range of 3 to 5, 75 MHz and can be calculated by:

(EQ 4)

The recommended setting of the divider is 4.

The next step is to calculate the Division ratio:

(EQ 5)

The N-counter value of the Fractional N Synthesizer can be calculated with the formula:

N = whole numbered part of

(EQ 6)

The A-Counter is the rest of this Division and can be calculated by:

(EQ 7)

In case, the Value of the A-counter is zero, the value of the A-counter has to be increased by M and the value of N has to be decreased by one

AC = M

N = N - 1

(EQ 8)

In case, the N-counter is in the range from 10 to 25 and the value of the A-counter is less than N-counter-7:

INT < 3 : 0 > = N - 10

INT < 7 : 4 > = AC - 1

(EQ 9)

Note:

In case, no solution can be found for the wanted center frequency, it may help to modify the reference frequency or INT<8>

Table 24. P and M Values for Frequency Bands

Frequency Band

P

M

BSEL<1:0>

Note

315 MHz

1

4

0 (00)

300 – 320 MHz

433 MHz

1

5

1 (01)

425 – 450 MHz

868 MHz

2

5

2 (10)

865 – 870 MHz

915 MHz

2

5

3 (11)

902 – 928 MHz

f

RF

Pf

REF

M N

AC

FRAC

++

⋅

()

⋅⋅

=

f

REF

f

XTAL

divider

-------------------

=

DR

f

RF

Pf

REF

⋅

------------------

DR

INT

DR

FRAC

+

==

DR

INT

3

–

()

M

-----------------------------

AC

DR

INT

3

–

() NM

×

–

=